Non-destructive evaluation (NDE) has been crucial to inspecting buried material defects and monitoring operational safety of key structures such as airplane wing splices, automobile engines, and oil pipelines. Accurate identification and characterization of material defects such as fatigue flaws, surface and subsurface cracks, pitting, and stress or corrosion induced damage precursors (e.g. material property inconsistency) is of great importance to avoiding unexpected sudden failure of structures. Eddy current based NDE techniques are well known for inspection of conductive materials in the industry. The inspection is carried out by sensing the variation of out-of-surface magnetic field resulted from the disturbance of eddy currents by the defects. In conventional eddy current probes, an excitation coil is used to produce a magnetic field which is able to induce an eddy current in the material that is being inspected, and another circularly wound inductive coil is employed as the receiving coil to detect the out-of-surface magnetic field. However, the spatial resolution of such probes is severely limited by the size of the receiving coil, which has been a significant drawback.
The discovery of magnetoresistive effect and the development of micro and nanofabrication technologies have led to an advancement of miniaturized solid-state magnetic field sensors, such as giant magnetoresistive (GMR) sensors and tunnel magnetoresistive (TMR) sensors, which show high sensitivity to magnetic field and can be fabricated as small as tens of micrometers. Replacing the conventional receiving coils by such magnetic field sensors offers significantly higher inspection accuracy and can effectively boost the spatial resolution. However, one of the challenging situations for adapting magnetic field sensors in eddy current probes is that they cannot discriminate the sensed magnetic field as can the conventional circularly wound inductive coil, if the magnetic field produced directly by the excitation coils can also be detected by the sensors in addition to the field of interest produced by the eddy current that is to be measured. Such magnetic field produced by the excitation coils can be deemed a background noise field, which deviates magnetic field sensors away from their most linear region and lowers their detection accuracy. In worse case scenario, the background noise field saturates the magnetic field sensors and causes them to completely lose their sensitivity. Prior art (U.S. Pat. No. 7,952,348B2) employed planar spiral conductive coils/traces as the excitation coils and coupled magnetic sensors with their sensitivity axes in parallel to the surface of the test object for the purpose of avoiding the interference from its planar excitation coils, which intrinsically produce excitation magnetic fields out of and orthogonal to the coil plane. These planar excitation coils are naturally unfavourable to magnetic sensors with their sensitivity axes orthogonal to the test object surface. With the planar excitation coil, one faces a difficult situation of placement of magnetic sensors, by choosing either a weak magnetic field signal component in parallel to the object surface from the eddy current with the least interference from the excitation coil or the strongest magnetic field signal component orthogonal to the object surface from the eddy current with the most interference from the excitation coil. Either way the signal-to-noise ratio of this eddy current probe made with the planar spiral excitation coil is not ideal. Another instance, prior art (US20140312891A1) introduced an eddy current probe for tubular inspection, comprising two circular arrays of excitation coils that created a region without radial component of magnetic field. An array of radially sensitive magnetoresistive sensors were circularly placed in that region for the purpose of avoiding the interference from the excitation magnetic field. One of the disadvantages in that configuration is that the area being inspected is not in close proximity to the excitation coils. Therefore the eddy current produced is relatively weak, which also lowers the signal-to-noise ratio and weakens the sensitivity of the test probe. In addition, the prior art requires two driving modes with multiplexing circuitry in order to inspect axial and circumferential defects, which makes the probe structure cumbersome and operation somewhat complicated.